Mycorrhizal fungus communities of Douglas-fir (Pseudotsuga menziesii) seedlings and trees: effects of proximity to residual trees

Abstract

The influence of mature Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees in partially harvested stands on seedling growth and colonization by ectomycorrhizal fungi (EMF) is not well understood. To investigate this, Douglas-fir seedlings were planted near (<6 m) and far (>16 m) from 44- to 72-year-old residual Douglas-fir trees at three sites in western Washington, USA. Seedling stem growth was measured from 1998 through 2003 and root growth from 1998 to 2000. Growth of seedlings potted in soils gathered near each residual tree was also examined in a greenhouse at the University of Washington. Monthly soil cores were collected near residual trees, and residual tree roots were sampled by partially excavating the root system. Colonized roots were examined by morphotyping, PCR-RFLP, and sequence analysis of the internal transcribed spacer (ITS) and other ribosomal RNA genes.Seedlings <6 m from residual trees had significantly higher root to shoot biomass ratios. Stem growth was greater for seedlings <6 m from trees at a site with minimal understory, but slower at sites with more developed understory. EMF abundance was related positively to both root and stem growth in 1998 but was negatively related to stem growth in 1999.Seedlings <6 m from residual trees averaged 4.1 EMF taxa per seedling and had a total of 47 taxa compared to 3.3 taxa per seedling and 38 total taxa for >16 m seedlings. Rhizopogon spp. were dominant on both <6 m and >16 m seedlings. EMF taxa of residual trees, including Russula spp. (particularly R. nigricans), Tylospora spp., Tomentella spp., and Boletus zelleri, were more frequent on <6 m seedlings than on >16 m seedlings. The EMF community of >16 m seedlings was more similar to that of greenhouse seedlings.Russula nigricans, Boletus zelleri, Clavulina, and Rhizopogon parksii were common in soil cores. EMF abundance and diversity were highest at the less recently harvested site, and increased from 1998 to 1999 at all sites.Proximity to residual trees increases root growth and EMF abundance and diversity of seedlings. Residual trees may accelerate the establishment of mycorrhizal communities associated with mature forests.